Downhole tools using fluid as a driving force are increasingly used during downhole operations, especially for driving operational tools and/or for engagement of the borehole wall or borehole casing. Hydraulic power for these fluid working units is provided by downhole hydraulic pumps. Due to downhole conditions, such hydraulic pumps are limited in many ways and still have to perform efficiently to save time and money during downhole operations. The physical extent of the pumps is limited due to spatial restrictions in the borehole, the power supplied is limited, typically because a wireline reaching from the surface is limited due to large voltage drops over long distances, or if downhole batteries are used, the spatial restriction again becomes the limiting factor. Furthermore, hydraulic pumps must be efficient to provide sufficient driving force and speed for the downhole fluid working units since this limits downhole operating times, which in turn reduces cost. Also, downhole pumps must be durable since breakdowns are even more critical to operating times as all maintenance and repair must be done on the surface, necessitating a complete retraction of the downhole tools from the boreholes. Known hydraulic pumps comprise a plurality of piston chambers of cyclically varying volume in which the displacement of fluid through the piston chambers is provided by a rotating cam lobe forcing the pistons to move in a cyclic manner. However, such hydraulic pumps are often not sufficiently efficient to provide the power needed downhole and may furthermore suffer from wear on the moving parts.